The dev server makes extensive use of Socket,
however the App Engine environment does not allow all the capabilities
of native sockets. This filter will perform the toggle between using
App Engine sockets using the DevSocketImplFactory which maintains
a thread local flag for this purpose. This filter should be configured in
filter chain (here:
//java/com/google/appengine/tools/development/jetty9/webdefault.xml) just before
user code is entered into.